1. Field of the Invention
The present invention is broadly concerned with magnetic element temperature sensors, detectors for use with such sensors, closed-loop object treatment systems making use of the sensors and detectors to wirelessly determine a temperature parameter related to an object being treated, and corresponding methods. More particularly, the invention is concerned with closed-loop molding/heating/curing systems of the type used for the fabrication of composite parts such as aircraft and automotive components, and including wireless temperature parameter sensor/detector devices. Such systems include pressurized autoclaves, non-pressurized oven-type systems, and resin molding systems.
2. Description of the Prior Art
Autoclave molding is a modification of known pressure-bag and vacuum-bag molding techniques. The advanced autoclave composite process produces denser, void-free moldings owing to the more uniform and controlled heating and more uniform and controlled pressure conditions employed for part curing. It is widely used in the aerospace industry to fabricate high strength/weight ratio parts from pre-impregnated high strength fibers, such parts used in aircraft, spacecraft and missiles. Autoclaves are essentially heated pressure vessels, usually equipped with vacuum systems, and are designed to receive composite lay-ups on or in internal molds. Such lay-ups are then heated and cured within the autoclave to yield the finished parts. Curing pressures are generally in the range of 50 to 150 psi and cure cycle times normally involve many hours. The autoclave method accommodates higher temperature matrix resins such as epoxies, having higher strength properties as compared with conventional resins.
Resin transfer molding (RTM) is a low-pressure, low-emission closed molding process for moderate volume production quantities, filling the gap between the slow, contact molding processes and the faster, compression molding processes requiring higher tooling costs. In RTM, continuous strand mats and woven reinforcement are laid up dry in the bottom mold half. Preformed glass reinforcements are often used for complex mold shapes. The mold is then closed and clamped, and a low viscosity, resin and catalyst mixture is pumped into the mold, displacing the air through strategically located vents. Metered mixing equipment is used to control the resin/catalyst ratios, and the resin and catalyst are mixed using a motionless/static mixer, and is then injected into the mold port. Common matrix resins include polyester, vinyl ester, epoxy, and phenolics. RTM moldings are of a uniform thickness and exhibit two finished sides. In order to optimize the surface finish of the parts, a gel coat may be applied to the mold surface prior to molding. High quality parts produced by RTM include automotive body parts, bathtubs, and containers.
Other types of ovens also are used to execute “pressure-bag” or “vacuum-bag” molding processes. These ovens employ hot air to cure composite parts that are typically placed within plastic bags or under thin plastic sheeting sealed to an adjacent tooling surface. This allows a vacuum to be drawn within the cavity defined by the plastic bag or the plastic sheeting and the tooling surface.
Furthermore, repair processes are often executed upon portions of uncured composite material (adjacent previously cured composite materials) that are enclosed within a chamber formed by thin plastic sheeting. Again, a vacuum is typically drawn within the chamber formed by the part and its encapsulating plastic “shield” or “bag” to remove air that can cause voids in the final cured repair. Heat is applied to the uncured composite material via many means such as resistive heating blankets, hot air, high intensity lighting, microwaves, and induction heating of the carbon fibers or particulates in the resin.
A persistent problem with all of the foregoing techniques is the need to accurately monitor the temperature of the parts during the heating, molding and/or curing cycles. In most production curing processes, only the temperature of the air and/or tool is monitored during curing, wherein the curing process follows a “recipe” of time and air/tool temperature that has been pre-determined by curing test parts that have embedded sensors within the parts so as to correlate part temperature with air/tool temperature and dwell time. If in the unusual case that temperature sensors are used to monitor part temperatures during present-day curing processes of production parts (often for the curing of very thick parts), they are typically either surface thermocouples applied to strategic surface locations or are traditional thermocouples that are embedded into non-essential flashings. In either case, these thermocouples must be physically connected to a monitoring system if simple temperature monitoring is the goal or to the autoclave, oven, or RTM control system for curing control purposes. Applying thermocouples to parts and connecting them to the monitoring and/or control systems is a complex and time-intensive process and can also compromise the pressure integrity of the vacuum bag and/or autoclave. Regardless of whether used in an autoclave, oven, or in a repair process, “pressure-bag” or “vacuum-bag” molding processes often make use of thermocouple leads which extend through the vacuum-bag apparatus to external monitoring electronics. The leads penetrating the vacuum-bag or sheeting often cause vacuum leaks which not only interfere with maintenance of desired vacuum levels, but can also allow moisture to pass through the vacuum bag or sheeting, leading to improper cures.
Accordingly, there is a need in the art for improved object treatment apparatus and methods including wireless temperature parameter sensing allowing real-time, non-contact monitoring of objects such as parts and part precursors within the chambers, in order to determine temperature parameters during the course of object treatment (e.g., heating, molding, and/or curing) and thereby permit control of the treatment apparatus using closed-loop feedback without the need for sensor leads of any kind. Furthermore, it would be advantageous if these sensors could be placed deep within thick parts and the wireless detector (reader) could be remote from the part so as not to disturb the curing process. Finally, it would be advantageous if the sensors could remain within the production part after cure for the life of the part without causing any structural degradation of the part.